tests: cover more MPTCP socket options

[matttbe]

There are some improvements that can be done on the tests side to cover more code in some areas (code coverage), e.g. the socket option part.

Some socket options have quite a bit of dedicated code on MPTCP side, and they are not covered, e.g. SO_LINGER, SO_TIMESTAMPING_NEW, TCP_CONGESTION, TCP_NOTSENT_LOWAT (there is a test, but currently disabled, linked to #476)

Others have less dedicated code, but still need to be covered, e.g. SO_DEBUG, SO_PRIORITY, SO_SNDBUF, SO_RCVBUF, SO_INCOMING_CPU, SO_BINDTODEVICE, SO_BINDTOIFINDEX, IPV6_V6ONLY, IPV6_FREEBIND, IP_FREEBIND, IP_BIND_ADDRESS_NO_PORT, IP_LOCAL_PORT_RANGE, TCP_DEFER_ACCEPT + the read of some other options, see the code coverage.

Probably best and easier to cover them with packetdrill tests.